For forming a laminated coating film by a coating method in which three kinds of coating materials are applied using a wet-on-wet technique and then baked, there has been a conventionally used method by which the laminated coating film as a whole is cured. In this method, thermosetting coating materials for forming layers constituting a laminated coating film are selected so that all the layers can be cured at the same heating temperature after all the coating materials are applied. However, the conventional coating method has a problem that the obtained laminated coating film is inferior in surface texture and gloss to that obtained by baking a lower layer and then applying and baking coating materials for forming an intermediate layer and an upper layer. In this connection, various methods have been proposed to improve the surface texture and the gloss of a laminated coating film.
For example, Japanese Unexamined Patent Application Publication No. 2004-275966 (PTL 1) discloses a method for forming a coating film, the method comprising: a step of successively applying an intermediate paint, abase paint, and a clear paint in a wet-on-wet manner; and a heating step including both a low-temperature heating stage (heating at a temperature which is 25 to 80% of a curing temperature for a time which is 5 to 30% of a curing time) and a high-temperature heating stage (heating at a temperature which exceeds 80% and is not more than 120% of a curing temperature for a time which is 30 to 130% of a curing time). However, in the case of the conventional method for forming a multilayer coating film as described in PTL 1 and an article coated by the method, the appearance qualities, such as surface texture (smoothness) and gloss of the laminated coating film are not necessarily sufficient, and it is difficult to improve the surface texture and gloss to the levels required for the appearance qualities of automobiles. In this respect, coated articles having better appearance qualities and better durability have been demanded for automobile steel plates and the like, and further improvement of the wet-on-wet coating method has been desired.